1. Field of the Invention
The present invention relates to a concentrating photovoltaic module and a concentrating photovoltaic power generating system.
2. Related Background Art
Attention has been focused heretofore on the photovoltaic power generating systems utilizing the solar cell modules as an energy source being safe for and no strain on the environment, but emphasis is recently being shifted to development of inexpensive solar cell modules with higher efficiency, in order to make them competitive over the conventional power generating means such as the thermal power generation and others even from the economical aspect.
From this respect, attention is recently being given to a concentrating photovoltaic module and a concentrating photovoltaic power generating system consisting of the concentrating photovoltaic module and a sun tracking device. In the ordinary photovoltaic power generating systems the solar cell module itself is fixed at a certain position, but the relation between Sun and Earth varies with time as a matter of course. Therefore, the fixed solar cell module can be located at the best angle relative to the sun only for a moment, and can be said to receive the solar energy at inappropriate angles during other periods of time. This can not apply only to directions of the sun (so called hour angles) from the solar cell module side but also to seasonal change of the sun route (declination change). In addition, the reflectance on the surface of the solar cell module also increases as the angle of incidence of sunbeams deviates from a normal to the solar cell module. The light receiving angle of the solar cell module is thus also inappropriate in this respect, which causes loss. Such loss is considered to sum up even to 20 to 30% of the energy to be received otherwise.
In order to eliminate the inappropriateness of the light receiving angle, the solar cell module needs to be always maintained at the best angle to the sun. This idea leads to a scheme of the photovoltaic power generating system of a sun tracking type and the tracking of the sun is expected to increase the electric power generated per year by 25% to 40%.
The concentrating photovoltaic power generating systems of the sun tracking type are also under research similarly toward reduction of unit cost of power generation. Since the concentrating photovoltaic power generating systems allow great saving of solar cells, which are most expensive among the components of the concentrating photovoltaic modules constituting the photovoltaic power generating systems, it becomes feasible to implement extremely great cost reduction.
As generally mentioned, generated voltage increases with increase in light intensity, so as to increase the rate of output energy to input energy, i.e., conversion efficiency. This results in yielding a greater output when compared with a configuration wherein solar cells are spread over the same area.
In order to draw this effect satisfactorily, it is necessary to construct the concentrating photovoltaic power generating system capable of concentrating light at a high magnification. This system requires the concentrating photovoltaic module having an optical system for efficiently concentrating the sunbeams.
A conventional means for concentrating the sunbeams is one as shown in FIG. 17 wherein a solar cell 204 is located at a position approximately coincident with the focal length of Fresnel lens 201 and parallel to the Fresnel lens 201, so as to concentrate sunbeams 107 impinging on the Fresnel lens 201, onto the solar cell 204.
In this method, however, spherical aberration, chromatic aberration, etc. of the Fresnel lens 201 makes it harder to concentrate the sunbeams 107 at a point with decrease in the F-number (=focal length÷aperture) of the Fresnel lens 201, which decreases light concentration efficiency. As a result, the focal length of the Fresnel lens 201 was not allowed to be decreased much. For this reason, the conventional concentrating photovoltaic modules increased their thickness, weight, and cost as compared with the flat panel type solar cell modules, and the tracking device for making the concentrating photovoltaic module track the sun also had to be constructed in large scale in consideration of tracking performance, wind endurance, and so on.
Taking the above into consideration, Japanese Patent Application Laid-Open No. 7-231111 suggests an example in which the entire module is compactified by subdividing (or downsizing) a concentrating lens. This is the example in which both the concentrating optical elements and solar cells are reduced to shorten the focal length while maintaining the F-number, as against the conventional concentrating photovoltaic modules. In this example, however, the number of solar cells increased with decrease of the size, so as to raise problems of decrease in efficiency due to a voltage drop in a series configuration of the solar cells, and increase of manufacturing cost.
U.S. Pat. No. 5,089,055 suggests an example in which a plurality of concentrating optical elements and a plurality of optical fibers are used to guide the sunbeams onto one solar cell. However, when exit faces of the optical fibers were directly arrayed in front of the solar cell, unevenness in quantity of light (hereinafter referred to as xe2x80x9clight quantity unevennessxe2x80x9d) among the individual optical fibers directly resulted in light quantity unevenness on the solar cell, which reduced the efficiency of the solar cell. In order to eliminate the light quantity unevenness in the individual optical fibers, it was necessary to use sufficiently long optical fibers. Since the permissible angle range of incidence of sunbeams was narrow on the input faces of the optical fibers, it was necessary to increase the F-number of the concentrating optical elements.
The expression xe2x80x9clight quantity unevenness on a solar cellxe2x80x9d as herein employed is intended to mean a state such that portions with strong light and portions with weak light are generated on the solar cell, which results in that the portions with strong light and the portions with weak light have optimal operating points different from each other, whereby the optimal operating point is shifted. Thus, there are cases where the shifted optimal operating point may be a value lower than an optimal operating voltage to be obtained with uniform light quantity, thereby lowering the conversion efficiency. Further, there are also cases where the light quantity unevenness may cause partial temperature rise to break the solar cell.
The above U.S. patent also describes an example wherein the plurality of optical fibers are once guided to a tubular light pipe, the sunbeams are mixed by making use of internal reflection inside the light pipe, and the mixed light is guided to the solar cell. However, since the light pipe has a different refractive index from that of the optical fibers, reflection loss occurs at the interface between them. There also occurs transmission loss, because the sunbeams are transmitted through the reflection inside the light pipe. Namely, aluminum or silver commonly used as a reflecting material of internal reflectors has the reflectance of 85 to 95% (in the visible region). While the sunbeams are guided to the solar cell through several reflections on the internal reflector, the energy of sunbeams decreases every reflection on the internal reflector, which posed a problem that the sunbeams were unable to be efficiently guided to the solar cell. It was also necessary to take deterioration of the reflecting material or the like into consideration.
The spherical aberration and chromatic aberration of the aforementioned Fresnel lens 201 hindered uniform irradiation of the solar cell 204 with the concentrated sunbeams 107 to cause degradation of efficiency of the solar cell 204 and increase of temperature at only local areas in certain cases, which resulted in damaging the solar cell 204. When the solar cell 204 was of multiple junction structure (in which a plurality of pn junctions made of several different materials were stacked in the traveling direction of light), the chromatic aberration caused dispersion of spectral distribution on the solar cell 204, resulting in greatly decreasing the conversion efficiency of the solar cell 204.
The expression xe2x80x9cdispersion of spectral distributionxe2x80x9d as herein employed is intended to mean one caused by chromatic aberration or the like, specifically a state such that dispersion depending on locations is generated in spectral distribution on a solar cell, thereby generating locations with more light of longer wavelengths or locations with more light of shorter wavelengths as compared to ordinary spectral distribution.
Since the Fresnel lenses generally used had the merits of smaller thickness, lighter weight, and lower cost than spherical (or aspherical) lenses having the same focal length, they have commonly been used, but they had lower concentration efficiencies than the spherical lenses, because a spherical surface (or an aspherical surface) was approximated by an uneven surface.
Each of a Fresnel lens with an uneven surface on the photoreceptive surface side (which will be referred to hereinafter as a front convex Fresnel lens) and a Fresnel lens with an uneven surface on the opposite side to the photoreceptive surface (which will be referred to hereinafter as a back convex Fresnel lens) had both merits and demerits, but they both were inferior to the spherical lenses (aspherical lenses).
Namely, while it was preferable to use the front convex Fresnel lens with small coma (a characteristic capable of guiding even sunbeams off the normal direction to the Fresnel lens to the solar cell) in consideration of relief of tracking accuracy of the tracking device, relief of forming accuracy of the concentrating photovoltaic module, utilization of circumsolar radiation, etc., dust was easier to build up on the photoreceptive surface and reflection loss occurred at uneven portions without provision of a sufficient focal length in the case of the front convex Fresnel lens, as compared with the back convex Fresnel lens.
Neither of the above-stated methods succeeded in efficiently concentrating the sunbeams on the solar cell without light quantity unevenness and dispersion of spectral distribution (hereinafter referred to as xe2x80x9cspectral distribution dispersionxe2x80x9d) and, particularly, in providing a compact and inexpensive concentrating photovoltaic module.
An object of the present invention is to concentrate the sunbeams on the solar cell without light quantity unevenness and spectral distribution dispersion and with high efficiency. Another object of the invention is to provide a compact, lightweight, inexpensive concentrating photovoltaic module, and a concentrating photovoltaic power generating system comprising the concentrating photovoltaic module and a tracking device.
The present invention was accomplished on the basis of the above-discussed recognition.
A concentrating photovoltaic module according to the present invention is a concentrating photovoltaic module comprising: a lightguide member having at least one exit face and a plurality of entrance faces; and at least one solar cell positioned immediately after the exit face of the lightguide member; wherein the lightguide member is comprised of a light transmissive, solid medium having no refractive-index-discontinuity portion and a surface of the lightguide member is smooth, and wherein the lightguide member makes sunbeams incident on the plurality of entrance faces, totally reflected on side faces, and emergent from the exit face. This allows the sunbeams with different characteristics (light quantity and spectral distributions) incident on all the entrance faces in the concentrating photovoltaic module to be combined and to be guided in a reduced state of light quantity unevenness and spectral distribution dispersion to the solar cell with high efficiency.
Namely, the sunbeams with different characteristics incident from all the entrance faces of the lightguide member each are repeatedly totally reflected on the smooth side faces of the lightguide member to be mixed, and thereafter they are together mixed in the same area inside the lightguide member. It is thus feasible to guide the sunbeams in the reduced state of light quantity unevenness and spectral distribution dispersion to the solar cell and thus to prevent the degradation of efficiency of the solar cell due to the light quantity unevenness and spectral distribution dispersion. The expression xe2x80x9chaving no refractive-index-discontinuity portionxe2x80x9d stated herein refers to the refractive index being almost single; specifically, dispersion is preferably within 1% and more preferably within 0.5%. More preferably, it refers to the refractive index being single (though inevitable dispersion in fabrication is permitted).
Other features and effects of the present invention will be described below in detail with reference to the drawings.